Remanent magnetization is familiar through everyday occurrence of
permanent
magnetism. Ferromagnetic substances may acquire or lose magnetization
under some circumstances. Thus ferromagnetic material which cools from a
high temperature to a
lower one while held in a magnetic field is very efficiently magnetized
(called thermo-remanent magnetization). Also, the growth of mineral
grains in the presence of a magnetic
field produces chemical-remanent magnetization. And the accretion of
magnetic particles in the presence of a magnetic field produces
depositional-remanent magnetization. The total magnetization of a
natural object is often a superposition of magnetization components
acquired through several processes over its lifetime. And these
different components can be later disentangled, giving some details
about the conditions prevailing under each acquired
magnetization. Virtually all meteorites carry natural remanent
magnetization (e.g.,
Levy & Sonett 1978).

How did our solar system form ? The answer lies in part within the
asteroidal belt, located about 3 Astronomical Units (~ 4.5 ×
108 km) from the Sun, and containing ~ 105 small
rocky planetesimals/asteroids. The asteroidal belt is the origin of many
meteorites found on Earth. Among the meteorites, the chondrites contain
abundant millimeter-sized silicate spherules (chondrules) which were
formed ~ 4.5 × 109 years ago within the solar nebula, and
have remained relatively unchanged since. Unravelling this record may
provide constraints on the type and duration of processes that occurred
within the solar nebula.

Chondrites are not entirely pristine, as a few processes may have
occurred in the solar nebula, changing the original primary
characteristics of chondrites (e.g.,
Brearly 1997).
Examples are the many alterations, either within the original solar
nebula before coalescence or accretion into an asteroid, or after
accretion within the interior of an asteroid.

The magnetic properties of meteorites are studied to know more about the
physical conditions in the early solar system. The meteorites could have
been magnetized during the accretion and cooling stages of the formation
of the Solar nebula.

2.1.1.1 Origin Estimates of the primordial
magnetizing fields (the
fields responsible for the remanence) have been made through
various techniques. Careful measurements of the magnetic field
properties of meteorites, based on the thermo-remanent
magnetization model, have revealed the primeval magnetic field
strength required to give the observed
remanent magnetization. The evidence seems to show that chondrules
(~ 1 mm in size) inside meteorites (~ 10 cm to 1 m in size)
were probably magnetized by the interplanetary
magnetic field. A predicted theoretical magnetic field in the early
solar nebula (~ 30 µT =
0.3 Gauss), which was inherited from an earlier interstellar cloudlet,
is about the correct
value needed to magnetize the carbonaceous chondrites. In addition to
chondrules, small interstellar grains (~ 1 µm in size) have
been discovered in meteorites (notably silicon
carbide SiC grains, graphite grains, and corundum
Al2O3 grains), The
distribution of their sizes follows a log-normal equation (e.g.,
Sandford 1996).

A possible magnetic field for the early solar nebula may have had a
dipolar shape
with a strength around 1 Gauss. The magnetic field lines could have been
perpendicular to
the elongated nebular disk, in a dynamo model with a differentially
rotating protosolar nebula (gas density 3 × 10-10
cm-3,
temperature ~ 200 K, magnetic field ~ 1 Gauss, diameter
~ 7 Astronomical Units, e.g.
Levy & Sonett 1978).
Such large early interplanetary magnetic
fields may have decayed with the dispersal of the early nebular gas, on
a time scale of 10 million years (e.g.,
Umebayashi & Nakano
1984).

2.1.1.2 Evolution In the case of a well-preserved
meteorite, such as the Allende meteorite which fell to Earth on 8
February 1969 in Mexico, paleomagnetism has shown
that its chondrules may have acquired their random remanent
magnetization before accretion into the meteorite. During or soon after
accretion into the meteorite, a
sulfidation event occurred which remagnetized most of the meteorite, but
a fraction of the pre-accretion remanent magnetism survived. A
subsequent shock slightly rotated the chondrules in the meteorite.

2.1.1.3 Chemistry Magnetic minerals in meteorites
are quite often different from
those in terrestrial rocks. Kamacite is by far the most abundant and the
most common magnetic mineral in meteorites. Others include tretataenite,
magnetite, and titanomagnetite.
Shu et al. (1997)
proposed a model where a magnetosphere in a high
magnetic state (inner disk radius located far from star) with low gas
temperature (500 K) would allow partial retention of Na and K in rocky
chondrules located in the protostellar
disk, while a magnetosphere in a low magnetic state (inner disk radius
located close to star) with high gas temperature (1500 K) would
evaporate Na and K and leave only Ca-Al
oxides and silicates in ordinary chondrites in the protostellar disk.

Caveat: a difficulty in meteorite magnetism is that nobody knows what
may have happened to the meteorites after their fall to
Earth. Generally, atmospheric entry in the
Earth affects a meteorite's magnetization only in the outer few
centimeters, and it does not interfere with identification of the inner
primordial magnetization (e.g.,
Levy & Sonett 1978).
Artificial magnets on Earth may have been used later to identify
meteorites - such contacts with artificial magnets could produce a large
remanent magnetization in some types of meteorites (e.g., ordinary
chondrites), but not in others (e.g., achondrites).
Shocks and heat in the absence of a magnetic field may demagnetize the
meteorites. A good review on these topics can be found in
Sugiura and Strangway
(1988).

Spacecrafts visit comets rarely, for only brief time intervals in their
flythroughs, and at different places along the cometary tails.
Near the nucleus of a comet, the general ubiquitous interplanetary
magnetic field (~ 50 µGauss at 1 AU) gets compressed by the
pressure of cometary static ions, to values
~ 50 nT (= 0.5 milliGauss) at 1 AU from the Sun (e.g.
Spinrad et al. 1994).
There is usually no need for an intrinsic cometary magnetic field
attached to the cometary nucleus; all
effects are extrinsic. Magnetic disturbances in the interplanetary
magnetic field, due to the presence of comet Halley, have been measured
by the spacecrafts Giotto
(Mazelle et al. 1995),
Vega I and Vega II
(Mikhajlov and Maslenitsyn
1995).

Big asteroids could be viewed as micro-planets. A few of them have been
surveyed at a distance by spacecrafts, and deviations of the
interplanetary magnetic fields have been
measured in their vicinity. The small radius of the asteroid does not
permit the setting up of a dynamo magnetic field. The magnetic moment of
the asteroid is weak, weak enough that
the magnetic field cannot set up a bow shock and cannot carve a
recognizable cavity against the solar wind ram pressure, but it may be
strong enough to generate a bow wave and
dispersive anisotropic MHD waves (e.g.
Baumgärtel et
al. 1997).
An asteroid generates
disturbances in the interplanetary plasma flow, launching whistler waves
that are swept downstream by the flowing plasma (e.g.,
Kivelson et al. 1995).
The interaction of the solar
wind flow with the asteroid may depend on the properties of the
asteroid, such as its
magnetization and its electrical conductivity. The interplanetary field
may become draped around the asteroid
(Wang & Kivelson,
1996).

2.1.3.1 Gaspra The Galileo spacecraft acquired data
in 1990 during its passage at 1600
km from Gaspra, consistent with a diversion of the interplanetary flow
by the asteroid 951 Gaspra (e.g.,
Baumgärtel et
al. 1994;
Kivelson et al. 1995).
It is thought that some remanent
magnetization, left over from the time of formation of the asteroid,
could create a somewhat chaotic magnetic field (perhaps like an
imperfect non-ideal line dipole).

Gaspra orbits at a mean distance of 2 AU from the Sun, where the
interplanetary
magnetic field strength is ~ 2 nT = 20 µGauss. Gaspra's
magnetic moment (= Bsurfr3surf) has been estimated around
1.5 × 1011 Gauss m3, predicting a
chaotic surface magnetic field around
Bsurf ~ 0.5 Gauss at a radius
rsurf ~ 7 km (e.g. the dipole model of
Baumgärtel et
al. 1994).
However, the dipole model also predicted a change of magnetic field
magnitude which was not observed in the data of the Galileo probe
(Wang & Kivelson,
1996).
Thus the real magnetic field on Gaspra may be random (not dipolar).

2.1.3.2 Ida The Galileo spacecraft acquired data in
1993 during its passage at 2400
km from Ida, consistent with a diversion of the interplanetary flow by
effects from the asteroid 243 Ida (e.g.,
Burnham 1994;
Kivelson et al. 1995).
The asteroid Ida (radius ~ 15 km)
may have revealed to the Galileo spacecraft a weak magnetic field,
probably remanent. Ida affects the magnetic field of the solar wind
sweeping past it. It is not yet known if a model
with a conducting Ida or a model with a magnetic moment for Ida could
produce the observed signature in the interplanetary flow
(Kivelson et al. 1995).

2.1.4.1. Earth's Moon The data for Earth's Moon do
not show a large scale golbal magnetic field, so the magnetic moment
< 1 × 1012 Gauss. m3 (e.g.,
Lin et al. 1998).
The radius of Earth's Moon is ~ 1740 km. The Moon is located at 60
Earth radii from the Earth's center. The magnetic field strength at the
equatorial surface is < 2 µGauss (e.g.
Kivelson et al. 1996b).
The solar wind normally flows virtually unimpeded to the lunar surface,
where it is absorbed.

2.1.4.2. Europa Europa, a large rock at 9 Jupiter
radii from Jupiter's center, seems
to have an extrinsic magnetic field induced by a current-carrying
ionosphere, maintained by Jupiter's background magnetic field of
strength ~ 420 nT (= 4.2 milliGauss), as seen by the Galileo probe
(Kivelson et al. 1997).
The data for Europa are consistent with some kind of passive magnetic
dipole, of strength ~ 9 × 1015 Gauss m3. The
radius of Europa is ~ 1570 km. The magnetic field strength at
the equatorial surface amounts to 240nT = 2.4 mGauss (e.g.
Kivelson et al. 1997).

2.1.4.3. Callisto The Galileo spacecraft detected a
small
enhancement of the field strength related to small changes in the jovian
plasma environment caused by Callisto's
presence. Internal magnetic anomalies in the crust of Callisto could
also affect the result, being more probable than an internal dynamo.